1. Field of the Invention
This invention relates to an improved process for preparing crystalline aromatic polyetherketones. More particularly, this invention relates to a process for producing crystalline aromatic polyetherketones having a high molecular weight and excellent heat resistance, chemical resistance, mechanical strength and the like, by using a particular solvent.
2. Description of the Prior Art
Crystalline aromatic polyetherketones having a structure comprising phenylene groups bonded via an ether group and a ketone group have been regarded as molding materials since these polymers have excellent heat resistance, chemical resistance and mechanical strength.
For producing the aromatic polyetherketones, a process has been known which comprises reacting a dialkali metal salt of bisphenol having a ketone group with a dihalogeno compound having a ketone group in the presence of an aromatic sulfone at 250.degree.-400.degree. C. (U.S. Pat. No. 4,010,147). Further, a process which comprises heating halophenol having a ketone group with an alkali metal carbonate in N-methylpyrrolidone, aliphatic sulfone or aromatic sulfone at 200.degree.-400.degree. C. (U.S. Pat. No. 4,113,699) has been also known.
In general, in order to produce polymers having a high molecular weight, it is necessary to conduct the reaction in a solvent which can dissolve the produced polymer. In the case of obtaining crystalline aromatic polyetherketones, since the polymer is insoluble in an ordinary solvent at low temperature, the polymerization must be conducted at high temperature. Therefore, it has been said that the selection of a polymerization solvent having good heat stability and a strong polarity sufficient to accelerate the polymerization and to dissolve the resulting polymer is important for the production of crystalline aromatic polyetherketones having a high molecular weight.
As the polymerization solvent, aliphatic sulfone, aromatic sulfone, N-methylpyrrolidone and the like have been employed as described above. However, even these solvents are insufficient for producing highly crystalline aromatic polyetherketones of high molecular weight. For example, when aromatic sulfone is employed as a solvent, unfavorable phenomena such as gelation and coloring often occur because the reaction must be conducted at a high temperature for a long time to obtain high molecular weight polymers having a high crystallinity and high melting point. Further, N-methylpyrrolidone and aliphatic sulfones such as sulfolane are thermally unstable and are inferior to aromatic sulfone as the polymerization solvent.
On the other hand, it had been attempted to obtain a high molecular weight polyetherketone-sulfone copolymer and polysulfone by using benzophenone as the polymerization solvent. But all the experiments were unsuccessful. Therefore, it has been considered that high polar solvents such as sulfone and sulfoxide must be used to produce polymers having a high degree of polymerization. Actually, a high molecular weight polyetherketone-sulfone copolymer having a reduced viscosity of 2.57 can be obtained by reacting 0.075 mole of dipotassium salt of bis-(4-hydroxyphenyl)ketone, 0.045 mole of bis-(4-chlorophenyl)ketone and 0.030 mole of bis-(4-chlorophenyl)sulfone in diphenylsulfone at 290.degree. C. for 17 hours while the copolymer having a reduced viscosity of only 0.14 is produced when benzophenone is used as the solvent instead of diphenylsulfone (U.S. Pat. No. 4,010,147).
Further, it has been reported that high molecular weight poly(aryl ethers) were obtained only in the so-called dipolar aprotic solvents such as dimethylsulfoxide and sulfolane while benzophenone (a reasonably polar solvent) was quite ineffective even at high temperature (J. Polymer Science, Part A-1, Vol. 5, 2379 (1967)).
Thus, there have been no reports that the high molecular weight polyketones were obtained in the aromatic ketones as the polymerization solvents.